基于微动方法研究五大连池火山区尾山火山锥浅层剪切波速度结构

Shallow shear-wave velocity structures under the Weishan volcanic cone in Wudalianchi volcano field by microtremor survey

为了研究五大连池火山区尾山火山锥浅层三维波速结构特征,在尾山火山锥附近区域布设了无线地震检波器密集台阵,记录连续地震背景噪声波形数据.基于微动方法（拓展空间自相关方法）提取了台站间2~5 Hz频率范围的Rayleigh面波相速度频散曲线.利用面波层析成像方法反演获得2~5 Hz Rayleigh面波二维相速度图像,基于每一个网格节点的频散曲线,进一步反演获得了尾山火山锥附近区域地表至700m深度的三维剪切波速度结构.成像结果显示：在0~150m较浅深度,靠近尾山火山锥区域显示为相对高速异常,远离火山锥区域则显示为相对低速异常.而至150~700m较深深度,波速异常特征与浅部相反,靠近尾山火山锥的区域显示为相对低速异常,远离火山锥的区域显示为相对高速异常.在远离尾山火山锥区域,浅层的相对低速异常可能与松散沉积层有关,深部的高速异常则反映了结晶变质岩的影响.在靠近尾山火山锥区域,浅部的相对高速异常应该反映了出露地表的玄武岩,而深部的相对低速异常则可能反映了火山通道周围广泛发育的破碎裂隙结构及其火山喷发后孔隙流体填充的影响.

A dense seismic array with wireless digital geophones was deployed to recordcontinuous seismic noise to investigate the shallow velocity structure beneath the Weishan volcanic cone.The ESPAC technique,which is one of popular microtremor survey methods,was adopted to extract the Rayleigh wave phase velocity dispersion curves between station pairs.We obtained high-resolution phase velocity maps at 2-5Hz with these dispersion measurements,and constructed a 3-D shallow shear-wave velocity model beneath Weishan volcanic cone by inverting the phase velocity dispersion curves at each grid points using an iterative linearized least-square inversion scheme.Significant velocity anomalies at different depths（0-700 m）and different regions are shown in the obtained 3-D shear-wave velocity model.At 0-150 mdepth,the low velocity anomalies are mainly distributed far away from the Weishan volcanic cone,and the high velocity anomalies are mainly distributed near the cone.On the contrary,at deeper depth of 150-700m,it was shown a totally different feature with the low velocity anomalies mainly constrained near the cone,and the high velocity anomalies situated far away from it.Field geological survey indicates that the Weishan volcanic areas are covered by unconsolidated sediment layer and very porous basalt.Therefore,far away from the cone the low velocity anomaly at shallow depth（〈150m）may correlate with the unconsolidated sediments,and high velocity anomaly at deeper depth（〉150m）implies the influence of the crystalline metamorphic rocks.Otherwise,near the Weishan volcanic cone area,the relatively high velocity anomaly at shallow depth may be due to the consequence of volcanic eruptions of porous basalt,while the relatively low velocity anomaly at deeper depth probably points to the volcanism caused more fragmentations which were filled with pore fluid.